HTTP State Management Mechanism
University of California, Berkeley
abarth@eecs.berkeley.eduhttp://www.adambarth.com/httpstateInternet-DraftThis document defines the HTTP Cookie and Set-Cookie headers.
These headers can be used by HTTP servers to store state on HTTP user
agents, letting the servers maintain a stateful session over the
mostly stateless HTTP protocol. The cookie protocol has many
historical infelicities and should be avoided for new applications of
HTTP.
NOTE: If you have suggestions for improving the draft, please send
email to http-state@ietf.org. Suggestions with test cases are
especially appreciated.This document defines the HTTP Cookie and Set-Cookie header. Using
the Set-Cookie header, an HTTP server can store name/value pairs and
associated metadata (called cookies) at the user agent. When the user
agent makes subsequent requests to the server, the user agent uses the
metadata to determine whether to return the name/value pairs in the
Cookie header.Although simple on its surface, the cookie protocol has a number of
complexities. For example, the server indicates a scope for each
cookie when sending them to the user agent. The scope indicates the
maximum amount of time the user agent should retain the cookie, to
which servers the user agent should return the cookie, and for which
protocols the cookie is applicable.For historical reasons, the cookie protocol contains a number of
security and privacy infelicities. For example, a server can indicate
that a given cookie is intended for "secure" connections, but the
Secure attribute provides only confidentiality (not integrity) from
active network attackers. Similarly, cookies for a given host are
shared across all the ports on that host, even though the usual
"same-origin policy" used by web browsers isolates content retrieved
from different ports.The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in document are to be
interpreted as described in .Requirements phrased in the imperative as part of algorithms (such
as "strip any leading space characters" or "return false and abort
these steps") are to be interpreted with the meaning of the key word
("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.This specification uses the Augmented Backus-Naur Form (ABNF)
notation of .The following core rules are included by reference, as defined in
, Appendix B.1: ALPHA (letters), CR (carriage
return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE
(double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space), HTAB (horizontal tab),
VCHAR (any visible [USASCII] character), and WSP (whitespace).The terms user agent, client, server, proxy, and origin server have
the same meaning as in the HTTP/1.1 specification.The terms request-host and request-URI refer to the values the user
agent would send to the server as, respectively, the host (but not
port) and abs_path portions of the absoluteURI (http_URL) of the HTTP
Request-Line.We outline here a way for an origin server to send state
information to a user agent, and for the user agent to return the
state information to the origin server.To initiate a session, the origin server includes a Set-Cookie header
in an HTTP response. (Note that "session" here does not refer to a
persistent network connection but to a logical session created from HTTP
requests and responses. The presence or absence of a persistent
connection should have no effect on the use of cookie-derived
sessions).The user agent returns a Cookie request header to the origin server
if it chooses to continue a session. The Cookie header contains a number
of cookies the user agent received in previous Set-Cookie headers. The
origin server MAY ignore the Cookie header or use the header to
determine the current state of the session. The origin server MAY send
the user agent a Set-Cookie response header with the same or different
information, or it MAY send no Set-Cookie header at all.Servers MAY return a Set-Cookie response header with any response.
User agents SHOULD send a Cookie request header, subject to other rules
detailed below, with every request.An origin server MAY include multiple Set-Cookie header fields in a
single response. Note that an intervening gateway MUST NOT fold multiple
Set-Cookie header fields into a single header field.[TODO: Put some examples here.This section describes the syntax and semantics of a well-behaved
profile of the protocol. Servers SHOULD use the profile described in
this section, both to maximize interoperability with existing user
agents and because a future version of the cookie protocol could remove
support for some of the most esoteric aspects of the protocol. User
agents, however, MUST implement the full protocol to ensure
interoperability with servers making use of the full protocol.The Set-Cookie header is used to send cookies from the server to
the user agent.Informally, the Set-Cookie response header comprises the token
Set-Cookie:, followed by a cookie. Each cookie begins with a
name-value-pair, followed by zero or more attribute-value pairs.
Servers SHOULD NOT send Set-Cookie headers that fail to conform to
the following grammar:Servers SHOULD NOT include two attributes with the same name.The cookie-value is opaque to the user agent and MAY be
anything the origin server chooses to send. "Opaque" implies that
the content is of interest and relevance only to the origin server.
The content is, in fact, readable by anyone who examines the
Set-Cookie header.To maximize compatibility with user agents, servers that wish to
store non-ASCII data in a cookie-value SHOULD encode that data using
a printable ASCII encoding, such as base64.NOTE: The syntax above allows whitespace between the attribute
and the U+003D ("=") character. Servers wishing to interoperate
with some legacy user agents might wish to elide this
whitespace.This section describes a simplified semantics of the Set-Cookie
header. These semantics are detailed enough to be useful for
understanding the most common uses of the cookie protocol. The full
semantics are described in .When the user agent receives a Set-Cookie header, the user agent
stores the cookie in its cookie store. When the user agent
subsequently makes an HTTP request, the user agent consults its
cookie store and includes the applicable, non-expired cookies in the
Cookie header.If the cookie store already contains a cookie with the same
cookie-name, domain-value, and path-value, the existing cookie is
evicted from the cookie store and replaced with the new value.
Notice that servers can delete cookies by including an Expires
attribute with a value in the past.Unless the cookie's attributes indicate otherwise, the cookie is
returned only to the origin server, and it expires at the end of the
current session (as defined by the user agent). User agents ignore
unrecognized cookie attributes.The Expires attribute indicates the maximum lifetime of the
cookie, represented as the date and time at which the cookie
expires. The user agent is not required to retain the cookie
until the specified date has passed. In fact, user agents often
evict cookies from the cookie store due to memory pressure or
privacy concerns.The Domain attribute specifies those hosts for which the cookie
will be sent. For example, if the Domain attribute contains the
value ".example.com", the user agent will include the cookie in
the Cookie header when making HTTP requests to example.com,
www.example.com, and www.corp.example.com. (Note that a leading
U+002E ("."), if present, is ignored.) If the server omits the
Domain attribute, the user agent will return the cookie only to
the origin server.
WARNING: Some legacy user agents treat an absent Domain
attribute as if the Domain attribute were present and contained
the current host name. For example, if example.com returns a
Set-Cookie header without a Domain attribute, these user agents
will send the cookie to www.example.com.The user agent will reject cookies (refuse to store them in the
cookie store) unless the Domain attribute specifies a scope for
the cookie that would include the origin server. For example, the
user agent will accept a Domain attribute of ".example.com" or of
".foo.example.com" from foo.example.com, but the user agent will
not accept a Domain attribute of ".bar.example.com" or of
".baz.foo.example.com".NOTE: For security reasons, some user agents are configured to
reject Domain attributes that do not correspond to a "registry
controlled" domain (or a subdomain of a registry controlled
domain). For example, some user agents will reject Domain
attributes of ".com".The Path attribute limits the scope of the cookie to a set of
paths. When a cookie has a Path attribute, the user agent will
include the cookie in an HTTP request only if the path portion of
the Request-URI matches (or is a subdirectory of) the cookie's
Path attribute, where the U+002F ("/") character is interpreted as a
directory separator. If the server omits the Path attribute, the
user agent will use the directory of the Request-URI's path
component as the default value.Although seemingly useful for isolating cookies between
different paths within a given domain, the Path attribute cannot
be relied upon for security for two reasons: First, user agents do
not prevent one path from overwriting the cookies for another
path. For example, if a response to a request for /foo/bar.html
attempts to set a cookie with a Path attribute of "/baz" the user
agent will store that cookie in the cookie store. Second, the
"same-origin" policy implemented by many user agents does not
isolate different paths within an origin. For example,
/foo/bar.html can read cookies with a Path attribute of "/baz"
because they are within the "same origin".The Secure attribute limits the scope of the cookie to "secure"
channels (where "secure" is defined by the user agent). When a
cookie has the Secure attribute, the user agent will include the
cookie in an HTTP request only if the request is transmitted over
a secure channel (typically TLS ).Although seemingly useful for protecting cookies from active
network attackers, the Secure attribute protects only the cookie's
confidentiality. An active network attacker can overwrite Secure
cookies from an insecure channel, disrupting the integrity of the
cookies.The HttpOnly attribute limits the scope of the cookie to HTTP
requests. In particular, the attribute instructs the user agent to
elide the cookie when providing access to its cookie store via
"non-HTTP" APIs (as defined by the user agent).The user agent returns stored cookies to the origin server in
the Cookie header. If the server conforms to the requirements in
this section, the requirements in the next section will cause the
user agent to return a Cookie header that conforms to the following
grammar:Each cookie-pair represents a cookie stored by the user
agent. The cookie-name and the cookie-value are returned verbatim
from the corresponding parts of the Set-Cookie header.Notice that the cookie attributes are not returned. In
particular, the server cannot determine from the Cookie header alone
when a cookie will expire, for which domains the cookie is valid,
for which paths the cookie is valid, or whether the cookie was set
with the Secure or HttpOnly attributes.The semantics of individual cookies in the Cookie header is not
defined by this document. Servers are expected to imbue these
cookies with server-specific semantics.For historical reasons, the full cookie protocol contains a number of
exotic quirks. This section is intended to specify the cookie protocol
in enough detail to enable a user agent that implements the protocol
precisely as specified to interoperate with existing servers.Conformance requirements phrased as algorithms or specific steps
may be implemented in any manner, so long as the end result is
equivalent. (In particular, the algorithms defined in this
specification are intended to be easy to follow, and not intended to
be performant.)This section defines a number of algorithms used by the cookie
protocol.The user agent MUST use the following algorithm to *parse a
cookie-date*:
Using the grammar below, divide the cookie-date into
date-tokens.
Process each date-token sequentially in the order the
date-tokens appear in the cookie-date:
If the found-day-of-month flag is not set and the date-token
matches the day-of-month production, set the found-day-of-month
flag and set the day-of-month-value to the number denoted by the
date-token. Skip the remaining sub-steps and continue to the
next date-token.If the found-month flag is not set and the date-token matches
the month production, set the found-month flag and set the
month-value to the month denoted by the date-token. Skip the
remaining sub-steps and continue to the next date-token.If the found-year flag is not set and the date-token matches
the year production, set the found-year flag and set the
year-value to the number denoted by the date-token. Skip the
remaining sub-steps and continue to the next date-token.If the found-time flag is not set and the token matches the
time production, set the found-time flag and set the hour-value,
minute-value, and second-value to the numbers denoted by the
digits in the date-token, respectively. Skip the remaining
sub-steps and continue to the next date-token.Abort these steps and *fail to parse* if
at least one of the found-day-of-month, found-month,
found-year, or found-time flags is not set,the day-of-month-value is less than 1 or greater than 31,the year-value is less than 1601 or greater than 30827,the hour-value is greater than 23,the minute-value is greater than 59, orthe second-value is greater than 59.If the year-value is greater than 68 and less than 100,
increment the year-value by 1900.If the year-value is greater than or equal to 0 and less than
69, increment the year-value by 2000.Let the parsed-cookie-date be the date whose day-of-month,
month, year, hour, minute, and second (in GMT) are the
day-of-month-value, the month-value, the year-value, the
hour-value, the minute-value, and the second-value,
respectively.Return the parsed-cookie-date as the result of this
algorithm.A *canonicalized* host-name is the host-name converted to
lower case.A request-host *domain-matches* a cookie-domain if at least one
of the following conditions hold:
The cookie-domain and the canonicalized request-host are
identical.The cookie-domain is a suffix of the canonicalized
request-host, the last character of the canonicalized
request-host that is not included in the cookie-domain is a U+002E
(".") character, and request-host is a host name (i.e., not an
IP address). [TODO: Is this the right way to spec this???]The user agent MUST use the following algorithm to compute the
*default-path* of a cookie:
Let uri-path be the path portion of the Request-URI.If the first character of the uri-path is not a U+002F ("/")
character, output U+002F ("/") and skip the remaining steps.If the uri-path contains only a single U+002F ("/") character,
output U+002F ("/") and skip the remaining steps.Output the characters of the uri-path from the first character
up to, but not including, the right-most U+002F ("/").A request-path *path-matches* a cookie-path if at least one of
the following conditions hold: [TODO: This isn't exactly what IE
or Firefox does.]
The cookie-path and the request-path are identical.The cookie-path is a prefix of the request-path and the last
character of the cookie-path is U+002F ("/").The cookie-path is a prefix of the request-path and the first
character of the request-path that is not included in the
cookie-path is a U+002F ("/") character.When a user agent receives a Set-Cookie header in an HTTP
response, the user agent *receives a set-cookie-string*
consisting of the value of the header.A user agent MUST use the following algorithm to parse
set-cookie-strings:
If the set-cookie-string is empty or consists entirely of WSP
characters, the user agent MAY ignore the set-cookie-string
entirely.If the set-cookie-string contains a U+003B (";") character:
The name-value-pair string consists of the characters up to,
but not including, the first U+003B (";"), and the
unparsed-attributes consist of the remainder of the
set-cookie-string (including the U+003B (";") in question).
Otherwise:
The name-value-pair string consists of all the characters
contained in the set-cookie-string, and the unparsed-attributes
is the empty string.If the name-value-pair string contains a U+003D ("=") character:
The (possibly empty) name string consists of the characters
up to, but not including, the first U+003D ("=") character, and
the (possibly empty) value string consists of the characters
after the first U+003D ("=") character.
Otherwise:
The name string is empty, and the value string consists of
the entire name-value-pair string.Remove any leading or trailing WSP characters from the name
string and the value string.The cookie-name is the name string, and the cookie-value is the
value string.The user agent MUST use the following algorithm to parse the
unparsed-attributes:
If the unparsed-attributes string is empty, skip the rest of
these steps.Consume the first character of the unparsed-attributes (which
will be a U+003B (";") character).If the remaining unparsed-attributes contains a U+003B (";")
character:
Consume the characters of the unparsed-attributes up to, but
not including, the first U+003B (";") character.
Otherwise:
Consume the remainder of the unparsed-attributes.
Let the cookie-av string be the characters consumed in this
step.If the cookie-av string contains a U+003D ("=") character:
The (possibly empty) attribute-name string consists of the
characters up to, but not including, the first U+003D ("=")
character, and the (possibly empty) attribute-value string
consists of the characters after the first U+003D ("=")
character.
Otherwise:
The attribute-name string consists of the entire cookie-av
string, and the attribute-value string is empty. (Note that this
step differs from the analogous step when parsing the
name-value-pair string.)Remove any leading or trailing WSP characters from the
attribute-name string and the attribute-value string.Process the attribute-name and attribute-value according to the
requirements in the following subsections.Return to Step 1.When the user agent finishes parsing the set-cookie-string,
the user agent *receives a cookie* from the Request-URI with name
cookie-name, value cookie-value, and attributes
cookie-attribute-list.If the attribute-name case-insensitively matches the string
"Max-Age", the user agent MUST process the cookie-av as follows.If the first character of the attribute-value is not a DIGIT or a
"-" character, ignore the cookie-av.If the remainder of attribute-value contains a non-DIGIT
character, ignore the cookie-av.Let delta-seconds be the attribute-value converted to an
integer.If delta-seconds is less than or equal to zero (0), let
expiry-time be the current date and time. Otherwise, let the
expiry-time be the current date and time plus delta-seconds
seconds.Append an attribute to the cookie-attribute-list with an
attribute-name of Expires (note the name conversion) and an
attribute-value of expiry-time.If the attribute-name case-insensitively matches the string
"Expires", the user agent MUST process the cookie-av as follows.Let the parsed-cookie-date be the result of parsing the
attribute-value as cookie-date.If the attribute-value failed to parse as a cookie date, ignore
the cookie-av.If the user agent received the set-cookie-string from an HTTP
response that contains a Date header field and the contents of the
last Date header field successfully parse as a cookie-date:
Let server-date be the date obtained by parsing the contents of
the last Date header field as a cookie-date.Let delta-seconds be the number of seconds between the
server-date and the parsed-cookie-date (i.e., parsed-cookie-date
- server-date).Let the expiry-time be the current date and time plus
delta-seconds seconds.
Otherwise:
Let the expiry-time be the parsed-cookie-date.If the expiry-time is later than the last date the user agent
can represent, the user agent MAY replace the expiry-time with the
last representable date.If the expiry-time is earlier than the first date the user agent
can represent, the user agent MAY replace the expiry-time with the
first representable date.Append an attribute to the cookie-attribute-list with an
attribute-name of Expires and an attribute-value of expiry-time.If the attribute-name case-insensitively matches the string
"Domain", the user agent MUST process the cookie-av as follows.If the attribute-value is empty, the behavior is undefined.
However, user agent SHOULD ignore the cookie-av entirely.If the first character of the attribute-value string is U+002E
("."):
Let cookie-domain be the attribute-value without the leading
U+002E (".") character.
Otherwise:
Let cookie-domain be the entire attribute-value.Convert the cookie-domain to lower case.[TODO: Test ".127.0.0.1" and "127.0.0.1"]Append an attribute to the cookie-attribute-list with an
attribute-name of Domain and an attribute-value of
cookie-domain.If the attribute-name case-insensitively matches the string
"Path", the user agent MUST process the cookie-av as follows.If the attribute-value is empty or if the first character of the
attribute-value is not U+002F ("/"):
Let cookie-path be the default-path. [TODO: We need more tests
for this, including with " characters and with multiple Path
attributes.]
Otherwise:
Let cookie-path be the attribute-value.Append an attribute to the cookie-attribute-list with an
attribute-name of Path and an attribute-value of cookie-path.If the attribute-name case-insensitively matches the string
"Secure", the user agent MUST append an attribute to the
cookie-attribute-list with an attribute-name of Secure and an empty
attribute-value.If the attribute-name case-insensitively matches the string
"HttpOnly", the user agent MUST append an attribute to the
cookie-attribute-list with an attribute-name of Secure and an empty
attribute-value.When the user agent receives a cookie, the user agent SHOULD
record the cookie in its cookie store as follows.A user agent MAY ignore a received cookie in its entirety if the
user agent is configured to block receiving cookies. For example, the
user agent might wish to block receiving cookies from "third-party"
responses.The user agent stores the following fields about each cookie: name,
value, expiry-time, domain, path, creation-time, last-access-time,
persistent-flag, host-only-flag, secure-only-flag, and
http-only-flag.When the user agent receives a cookie from a Request-URI with name
cookie-name, value cookie-value, and attributes cookie-attribute-list,
the user agent MUST process the cookie as follows:
Create a new cookie with name cookie-name, value cookie-value.
Set the creation-time and the last-access-time to the current date
and time.If the cookie-attribute-list contains an attribute with an
attribute-name of "Expires":
Set the cookie's persistent-flag to true.Set the cookie's expiry-time to attribute-value of the last
attribute in the cookie-attribute-list with an attribute-name of
"Expires". [TODO: Test that this really works when mixing
Max-Age and Expires.]
Otherwise:
Set the cookie's persistent-flag to false.Set the cookie's expiry-time to the latest representable
date.If the cookie-attribute-list contains an attribute with an
attribute-name of "Domain":
Let the domain-attribute be the attribute-value of the last
attribute in the cookie-attribute-list with an attribute-name of
"Domain".
Otherwise:
Let the domain-attribute be the empty string.If the user agent is configured to use a "public suffix" list
and the domain-attribute is a public suffix:
If the domain-attribute is identical to the canonicalized
Request-URI's host:
Let the domain-attribute be the empty string.
Otherwise:
Ignore the cookie entirely and abort these stepsNOTE: A "public suffix" is a domain that is controlled by a
public registry, such as "com", "co.uk", and "pvt.k12.wy.us".
This step is essential for preventing attacker.com from
disrupting the integrity of example.com by setting a cookie with
a Domain attribute of "com". Unfortunately, the set of public
suffixes (also known as "registry controlled domains") changes
over time. If feasible, user agents SHOULD use an up-to-date
public suffix list, such as the one maintained by the Mozilla
project at http://publicsuffix.org/.If the domain-attribute is non-empty:
If the Request-URI's host does not domain-match the
domain-attribute, ignore the cookie entirely and abort these
steps.Set the cookie's host-only-flag to false.Set the cookie's domain to the domain-attribute.
Otherwise:
Set the cookie's host-only-flag to true.Set the cookie's domain to the host of the Request-URI.If the cookie-attribute-list contains an attribute with an
attribute-name of "Path", set the cookie's path to attribute-value
of the last attribute in the cookie-attribute-list with an
attribute-name of "Path". Otherwise, set cookie's path to the
default-path of the Request-URI.If the cookie-attribute-list contains an attribute with an
attribute-name of "Secure", set the cookie's secure-only-flag to
true. Otherwise, set cookie's secure-only-flag to false.If the cookie-attribute-list contains an attribute with an
attribute-name of "HttpOnly", set the cookie's http-only-flag to
true. Otherwise, set cookie's http-only-flag to false.Remove from the cookie store all cookies that share the
same name, domain, path, and host-only-flag as the newly created
cookie. [TODO: Validate this list!] [TODO: There's some funny
business around http-only here.]If the cookie's name and value are both empty, abort these
steps.If the cookie's expiry-time is not in the future, abort these
steps.Insert the newly created cookie into the cookie store.The user agent MUST evict a cookie from the cookie store if, at any
time, a cookie exists in the cookie store with an expiry date in the
past.The user agent MAY evict a cookie from the cookie store if the
number of cookies sharing a domain field exceeds some predetermined
upper bound (such as 50 cookies).The user agent MAY evict a cookie from the cookie store if the
cookie store exceeds some predetermined upper bound (such as 3000
cookies).When the user agent evicts a cookie from the cookie store, the
user agent MUST evict cookies in the following priority order:
Cookies with an expiry date in the past.Cookies that share a domain field with more than a predetermined
number of other cookies.All cookies.If two cookies have the same removal priority, the user agent
MUST evict the cookie with the least recent last-access date
first.When "the current session is over" (as defined by the user agent),
the user agent MUST remove from the cookie store all cookies with the
persistent-flag set to false.When the user agent generates an HTTP request, the user agent
SHOULD attach exactly one HTTP header named Cookie if the
cookie-string (defined below) for the Request-URI is non-empty.A user agent MAY elide the Cookie header in its entirety if the
user agent is configured to block sending cookies. For example, the
user agent might wish to block sending cookies during "third-party"
requests.The user agent MUST use the following algorithm to compute the
cookie-string from a cookie store and a Request-URI:
Let cookie-list be the set of cookies from the cookie store
that meet all of the following requirements:
Let request-host be the Request-URI's host. Either:
The cookie's host-only-flag is true and the canonicalized
request-host is identical to the cookie's domain.
Or:
The cookie's host-only-flag is false and the request-host
domain-matches cookie's domain.The Request-URI's path patch-matches cookie's path.If the cookie's secure-only field is true, then the
Request-URI's scheme must denote a "secure" protocol (as defined
by the user agent).
NOTE: The notion of a "secure" protocol is not defined by
this document. Typically, user agents consider a protocol
secure if the protocol makes use of transport-layer security,
such as TLS. For example, most user agents consider "https"
to be a scheme that denotes a secure protocol.If the cookie's http-only field is true, then exclude the
cookie unless the cookie-string is being generated for an
"HTTP" API (as defined by the user agent).Sort the cookie-list in the following order:
Cookies with longer paths are listed before cookies
with shorter paths.Among cookies that have equal length path fields, cookies
with earlier creation-times are listed before cookies with later
creation-times.Update the last-access-time of each cookie in the cookie-list
to the current date and time.Serialize the cookie-list into a cookie-string by processing each
cookie in the cookie-list in order:
If the cookie's name is non-empty, output the cookie's
name followed by the U+003D ("=") character.Output the cookie's value.If there is an unprocessed cookie in the cookie-list, output
the characters U+003B and U+0020 ("; ").Practical user agent implementations have limits on the number
and size of cookies that they can store. General-use user agents
SHOULD provide each of the following minimum capabilities:
At least 4096 bytes per cookie (as measured by the sum of the
length of the cookie's name, value, and attributes).At least 50 cookies per domain.At least 3000 cookies total.Servers SHOULD use as few and as small cookies as possible to avoid
reaching these implementation limits and to avoid network latency due
to the Cookie header being included in every request.Servers should gracefully degrade if the user agent fails to return
one or more cookies in the Cookie header because the user agent might
evict any cookie at any time on orders from the user.One reason the cookie protocol uses such an esoteric syntax is
because many platforms (both in servers and user agents) provide
string-based application programmer interfaces (APIs), requiring
application-layer programmers to generate and parse the syntax used by
the cookie protocol.Instead of providing string-based APIs to the cookie protocols,
implementations would be well-served by providing more semantic APIs.
It is beyond the scope of this document to recommend specific API
designs, but there are clear benefits to accepting a abstract "Date"
object instead of a serialized date string.The cookie protocol is NOT RECOMMENDED for new applications.For applications that do use the cookie protocol, servers SHOULD
NOT rely upon cookies for security.For servers that do use cookies for security, servers SHOULD use a
redundant form of authentication, such as HTTP authentication or TLS
client certificates.A server that uses cookies to authenticate users can suffer
security vulnerabilities because some user agents let remote parties
issue HTTP requests from the user agent (e.g., via HTTP redirects and
HTML forms). When issuing those requests, user agent attaches cookies
even if the entity does not know the contents of the cookies, possibly
letting the remote entity exercise authority at an unwary server. User
agents can mitigate this issue to some degree by providing APIs for
suppressing the Cookie header on outgoing requests.Although this security concern goes by a number of names (e.g.,
cross-site scripting and cross-site request forgery), the issue stems
from cookies being a form of ambient authority. Cookies encourage
server operators to separate designation (in the form of URLs) from
authorization (in the form of cookies). Disentangling designation and
authorization can cause the server and its clients to become confused
deputies and undertake undesirable actions.Instead of using cookies for authorization, server operators might
wish to consider entangling designation and authorization by treating
URLs as object-capabilities. Instead of storing secrets in cookies,
this approach stores secrets in URLs, requiring the remote entity to
supply the secret itself. ALthough this approach is not a panacea,
judicious use of these principles can lead to more robust
security.Unless sent over a secure channel (such as TLS), the information in
the Set-Cookie and Cookie headers is transmitted in the clear.
All sensitive information conveyed in these headers is exposed to
an eavesdropper.A malicious intermediary could alter the headers as they travel
in either direction, with unpredictable results.A malicious client could alter the Cookie header before
transmission, with unpredictable results.Servers SHOULD encrypt and sign their cookies when transmitting
them to the user agent (even when sending the cookies over a secure
channel). However, encrypting and signing cookies does not prevent an
attacker from transplanting a cookie from one user agent to
another.In addition to encrypting and signing the contents of every
cookie, servers that require a higher level of security SHOULD use the
cookie protocol only over a secure channel.Cookies do not provide isolation by port. If a cookie is readable
by a service running on one port, the cookie is also readable by a
service running on another port of the same server. If a cookie is
writable by a service on one port, the cookie is also writable by a
service running on another port of the same server. For this reason,
servers SHOULD NOT both run mutually distrusting services on different
ports of the same host and use cookies to store security-sensitive
information.Cookies do not provide isolation by scheme. Although most commonly
used with the http and https schemes, the cookies for a given host are
also available to other schemes, such as ftp and gopher. This lack of
isolation is most easily seen when a user agent retrieves a URI with a
gopher scheme via HTTP, but the lack of isolation by scheme is also
apparent via non-HTTP APIs that permit access to cookies, such as
HTML's document.cookie API.Cookies do not provide integrity guarantees for sibling domains
(and their subdomains). For example, consider foo.example.com and
bar.example.com. The foo.example.com server can set a cookie with a
Domain attribute of ".example.com", and the user agent will include
that cookie in HTTP requests to bar.example.com. In the worst case,
bar.example.com will be unable to distinguish this cookie from a
cookie it set itself. The foo.example.com server might be able to
leverage this ability to mount an attack against bar.example.com.Similarly, an active network attacker can inject cookies into the
Cookie header sent to https://example.com/ by impersonating a response
from http://example.com/ and injecting a Set-Cookie header. The HTTPS
server at example.com will be unable to distinguish these cookies from
cookies that it set itself in an HTTPS response. An active network
attacker might be able to leverage this ability to mount an attack
against example.com even if example.com uses HTTPS exclusively.Servers can partially mitigate these attacks by encrypting and
signing their cookies. However, using cryptography does not mitigate
the issue completely because an attacker can replay a cookie he or she
received from the authentic example.com server in the user's session,
with unpredictable results.The cookie protocol relies upon the Domain Name System (DNS) for
security. If the DNS is partially or fully compromised, the cookie
protocol might fail to provide the security properties required by
applications.
Key words for use in RFCs to Indicate Requirement Levels
Harvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keywordIn many standards track documents several words are used to
signify the requirements in the specification. These words are
often capitalized. This document defines these words as they
should be interpreted in IETF documents. Authors who follow these
guidelines should incorporate this phrase near the beginning of
their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in RFC 2119.Note that the force of these words is modified by the
requirement level of the document in which they are used.Hypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.org
Augmented BNF for Syntax Specifications: ABNF
Brandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.net
The Transport Layer Security (TLS) Protocol Version 1.2
This document borrows heavily from RFC 2109. [TODO: Figure out the
proper way to credit the authors of RFC 2109.]